In the soft fruit industry (e.g. strawberries, peaches, plums, bananas and tomatoes), substantial losses are incurred during transport, storage and marketing because of the susceptibility of softer fruit to mechanical damage and invasion by microorganisms. To limit such losses, plant breeders have selected lines which are less soft, and for many fruits firm cultivars now dominate national and international trade. Unfortunately, while marketing losses may be lower when fruit is less soft, the firmer lines may have reduced market appeal and also tend to be deficient in flavour.
The flavour of ripe fruit is a function of a number of components including sweetness (sugars), sourness (organic acids) and volatile compounds. A large number of volatiles have been identified as contributing to flavour and aroma (1-8). In ripening tomato fruit, approximately 400 volatile compounds have been found in the ripening fruit (5) but of these only a small number have been identified as important components of flavour and aroma. These include Z-3-hexenal, Z-3-hexenol, 2-E-hexenal, hexanal, 3-methylbutanal, 3-methylbutanol, .beta.-ionone, 1-penten-3-one, 2-isobutylthiazole, 6-methyl-5-hepten-2-one, methyl salicylate, geranylacetone, E-2-heptenal, isobutyl cyanide and 2-phenylethanol (9,10,3), Flavour volatiles are formed by several different pathways such as the deamination and decarboxylation of amino acids (3-methylbutanal (11)) and lipid oxidation of unsaturated fatty acids (hexanal and the hexenals (12)) (FIG. 1).
The tomato alcohol dehydrogenase II enzyme (ADH 2; alcohol:NAD.sup.+ oxidoreductase; EC 1.1.1.1) has been implicated in the interconversion of the aldehyde and alcohol forms of these volatiles (13,14). The enzyme has also been shown to accumulate in the fruit during ripening and to have appropriate substrate specificities in vitro (14-16). Further, the activity of ADH2 in ripening tomato fruit appears to be a function of the softness of the fruit. For example, in the typically firm tomato fruit of commercial cultivars, ADH2 activity is low and this may contribute to the perception of poor flavour in these fruit.
The present inventors have sought to improve tomato fruit flavour by increasing the ADH2 activity in ripening tomato fruit through the introduction of an additional ADH2-encoding DNA sequence(s) into the parent plants. This involved, firstly, isolating a cDNA (see FIG. 1A) encoding ADH2 and then transforming tomato plant explants. The present inventors have now produced a number of tomato plants so transformed and have demonstrated significant changes to the ratios of 2-carbon aldehydes to alcohols. Whether directly or indirectly, these changes have altered the aroma and/or flavour of the fruit produced by transformed plants. Thus, transformed fruit plants with altered ADH2 activity may have direct commercial value and provide valuable stocks for breeding programs.